1. Field of the Invention
The present invention relates generally to the field of communications systems and more particularly to a device and method for increasing spectral efficiency, capacity and/or dispersion-limited reach in a communications system.
2. Related Art
Fiber optic technology has been widely adopted for use in communication systems due to its superiority over conventional copper cable in terms of speed, bandwidth capacity, signal quality, insensitivity to electromagnetic interference and data security, among other advantages. Since the first systems were deployed in the 1980s, per channel capacity has continuously increased from 155 Mb/s up to the current standard of 10 Gb/sec and further up to the newest systems with capacities of 40 Gb/sec. Although optical communication is among the fastest high-capacity data transport means currently available, there remain a number of factors that limit the fiber capacity. Some of these limitations are attributable to the point-to-point links connecting electrical switches or routers and the active devices that transmit and receive light at the ends of the fiber. Another factor is simple attenuation of the transmitted signal. In addition, various dispersion phenomena affect the ability to recover the signal.
Dispersion in uncompensated fiber optic links causes pulse broadening and intersymbol interference (“ISI”). In ISI, some of the energy transmitted for one bit overlaps at the receiver with that for other, typically adjacent, bits. If the electrical signal within the receiver is represented using an eye diagram, the dispersion can be seen to lead to eye closure. The dispersion can be chromatic dispersion and/or polarization mode dispersion (“PMD”). The degradation due to dispersion increases with signal bandwidth. In long distance transmission systems, dispersion can also interact with non-linearity in the optical fiber to further impair transmission.
In a typical optical transmission system, the optical carrier, generated by a laser source, is intensity-modulated with the data signal. Then, the signal is modulated using a directly on-off keyed (“OOK”) signal representing binary digits. The most commonly used data modulation format in optical systems has been a non-return to zero on-off keying (“NRZ-OOK”).
In an NRZ-OOK format, a binary ‘one’ is represented by light being ‘on’ and a binary ‘zero’ by light being ‘off’ This format generally exhibits good spectral efficiency in multi-wavelength systems, reasonable distance capability, and straightforward implementation. When longer transmission distances are required, such as in a submarine or in a long-haul terrestrial fiber-optic link, the NRZ-OOK format is often modified by returning the ‘one’ level to ‘zero’ within each bit period (RZ-OOK), and possibly by adding some amount of optical phase modulation to each bit. This modulation format increases the distance reach at the expense of more complicated components and reduced spectral efficiency.
The dispersion limited distance for NRZ format is approximately 80-100 km. One method for compensating for chromatic dispersion-induced ISI is within the optical domain, using relatively costly and bulky optical means. In 2001, electronic dispersion compensation (“EDC”) was suggested for extraction of information from ISI corrupted optical links. EDC has been shown to be more flexible and less expensive than true optical dispersion compensation. Using transversal filters, portions of the electronic input signal are subjected to different time delays and recombined after amplification to suitable levels. If the settings are carefully optimized, EDC can significantly improve the signal quality, however, the full potential of true optical dispersion compensation cannot be reached. For a given transmitter and fiber, electronic dispersion compensation can increase the achieved transmission distances on the order of fifty percent.
The dispersion limited reach with EDC is ultimately limited by the amount of spreading of the optical waveforms. For example, it has been shown that EDC links operating at 10 Gb/s are limited to approximately 200 km. The limit is imposed by the increased complexity of the equalizer, which grows exponentially with the span of ISI.
A number of modulation formats have been introduced in efforts to extend dispersion limited reach and/or spectral efficiency, with or without EDC. Such advanced modulation formats include Duobinary Modulation, Single Sideband Modulation, Duobinary Single Sideband Modulation, Quadrature phase shift keying (“QPSK”) or M-ary PSK, and Optical oQPSK. Other known methods include EDC performed on regular OOK systems (NRZ, RZ, CS-RZ, CRZ etc.), or on one of the above-described advanced modulation formats. Each of these approaches adds complexity and expense to the transmission system, and can also impact robustness, reliability and upgradeability. Accordingly, the need remains for a system and method that increases spectral efficiency, capacity and/or dispersion limited reach using inexpensive, flexible and robust means. The present invention is directed to such a need.